Display is a process of converting an electrical signal (data information) into visible light (visual information), and devices for realizing display, i.e., Man-Machine Interface (MMI) and Flat Panel Display (FPD), are currently the most popular class of display devices. In FPD, Liquid Crystal Display (LCD) is the earliest developed and commercialized product. At present, thin film transistor liquid crystal displays (TFT-LCD) have become a mainstream product in LCD applications.
The development of TFT-LCD has gone through a long period of basic research, and after achieving large-scale production and commercialization, TFT-LCD products are made larger in size and wider in application due to their advantages of thinness, being environmentally friendly, high performance, etc. TFT-LCD applications can be seen everywhere, whether in small-sized mobile phone screens or large-sized notebook PCs or monitors and large-sized liquid crystal televisions (LCD-TV). Early commercial TFT-LCD products basically uses a Twisted Nematic (TN) display mode, and the largest problem thereof is that the viewing angle is not large enough. With the increase in the size of TFT-LCD products, especially for the application of TFT-LCD in the TV field, an In-Plane Switching (IPS) display mode with a wide viewing angle characteristic has been developed and utilized. The IPS display mode was first published in a paper in 1974 by American R. Soref, and German G Baur proposed the application of IPS as a wide viewing angle technique to TFT-LCD. In 1995, Hitachi, Japan developed the first 13.3-inch IPS mode wide viewing angle TFT-LCD product in the world. Korean Hyundai Corporation has developed a Fringe Field Switching (FFS) display mode TFT-LCD product on the basis of IPS.
TFT-LCD is a liquid crystal display device under the control of a TFT switch, and the electrical and optical characteristics of liquid crystals directly affect the display effect. Different types of liquid crystals have different electrical and optical characteristics and different display modes. Performance parameters that have a larger influence on liquid crystal materials used for TFT-LCD include: a working temperature range, a driving voltage, a response speed, a contrast ratio, a hue, a tone, a viewing angle, etc., wherein the driving voltage is more affected by the dielectric constant anisotropy and the elastic coefficient, and the viscosity and elastic coefficient affect the response speed of a liquid crystal material, and the phase difference and refractive index anisotropy affect the hue of the liquid crystal display. In the past, those cyano-containing compounds cannot satisfy these conditions, and only fluorine-containing liquid crystal materials are applicable for the manufacture of TFT-LCDs.
In addition, one kind of liquid crystal molecules cannot meet all the requirements of TFT-LCD display, and a combination of many kinds of liquid crystal molecules is necessary. By combining many kinds of liquid crystal molecules, various physical property requirements for liquid crystal materials can be achieved, and these requirements mainly include 1) a high stability. 2) a moderate birefringence. 3) a low viscosity. 4) a larger dielectric anisotropy. 5) a wide temperature range. The ideal storage temperature range is −40° C. to 100° C. and in the case of special applications such as vehicle display, the temperature may be widened to −40° C. to 110° C.
Nowadays, technologies for LCD products have been very mature, successfully solving the technical difficulties of a viewing angle, a resolution, a colour saturation, a brightness, etc., and the display performance thereof has approached or exceeded those of CRT displays. Large-sized and medium-sized LCDs have gradually occupied the mainstream position of flat panel displays in the respective fields thereof. In order to pursue higher performance specifications, accelerating response time has become the goal pursued by various device manufacturers.
Specifically, the response time of a liquid crystal is limited by the rotary viscosity γ1/elastic constant K of the liquid crystal, and therefore from the viewpoint of the liquid crystal material, it is necessary to try to reduce the rotary viscosity γ1 of the liquid crystal medium while increasing the elastic constant K to achieve an accelerated response time. Furthermore, it is found in actual researches that the rotary viscosity and elastic constant are a pair of contradictory parameters; lowering the rotary viscosity causes the elastic constant to decrease, leading to a failure of achieving the target of reducing the response time. For devices, the goal of accelerating response time can be achieved by reducing cell thickness d, and this is easy to implement; however, since the delayed amount Δnd of a device is fixed, it is required to increase the optical anisotropy Δn thereof in terms of liquid crystal material for reducing cell thickness d.
Therefore, in order to meet the above-mentioned requirements, it is necessary to develop a series of compounds with superior performance to solve the problem of the liquid crystal display having a low response time.